Radar systems may be used for a variety of purposes, including; detection, tracking and imaging of one or more targets. Common to conventional radar systems are the ability to generate a radio frequency (RF) signal pulse with short time duration, reception of the return signal due to the pulse being reflected by a target(s), and an antenna for directing the RF pulse in a known direction, and receiving the return signal from a known direction.
Most radar systems are mono-static, the antenna for transmitting the RF pulse and receiving the return signal from the target(s) is either the same antenna, or two co-located antennas. Multi-static radar systems typically employ one or more pulse transmitters, and multiple return signal receivers, all of which are geographically dispersed. The separation distances between components in a multi-static radar system is a function of the desired detection range and angular resolution of return signals from distinct targets, or multiple targets which are closely grouped.
Further, the resolution and accuracy of a multi-static radar system is determined by how closely the components are time-synchronized. The need for time-synchronization is inherent in the ranging aspect of radar, to determine the distance between the receiving antenna and the target(s). Hence, in a multi-static radar system the receivers must be time-synchronized with the pulse transmitter(s) in order to accurately measure the overall time of flight (ToF) of the pulse, from the transmitter, to the target and back to the receiver.
Some multi-static radar systems employ wired connections between components for the purpose of distributing a time-synchronizing signal. Other multi-static radar systems employ a wireless link to convey such a signal with a modulated carrier frequency for the same purpose. Both of these methods achieve an accuracy of time-resolution that is limited by the modulation scheme, distribution method, and subsequent signal dispersion.
Conventional multi-static radar systems also rely on directional antennas, each of which has a known orientation in azimuth and elevation at both the transmitter and receiver. The azimuth and elevation information is required to determine the direction for transmitting a pulse, and the direction from which a return signal from a target(s) was received.
In addition to the azimuth and elevation information about the antenna(s), a multi-static radar system may also rely on geographical coordinates of each antenna as well, to determine the absolute location for each target(s).
There exists a class of multi-static radar applications for which many of the typical characteristics are either not present, or not readily available. In particular, wireless sensor networks might be composed of two or more physically small nodes, each having both a data communications and radar capability. Further, each node may have an arbitrary physical location and antenna orientation, resulting from the manner in which the nodes are emplaced. Due to the ad hoc, or unpredictable, relationship between nodes in such a wireless sensor network, certain approaches must be taken to ensure the sensor network is able to achieve data communications between and among nodes, and the multi-static radar capability is operational.
In particular, due to the variable orientation of each sensor node, relative to other nodes, an omni-directional or isotropic antenna is required for data communications. Further, each sensor node is typically battery-powered, and must use energy in the most efficient manner possible in order to achieve a useful lifespan. The limited energy available from a battery typically requires the sensor nodes to use relatively low-power radios for data communications and the multi-static radar function. Hence, the lower-power radio propagation range results in physical separation distances between nodes that is typically less than for a conventional multi-static radar system.
In order to detect, track and image target(s) in a given region, multiple sensor nodes may need to collaborate, which requires close time-synchronization. Further, energy efficiency can be improved by using a single radio for both the data communications and radar functions.
A typical wireless communication system is composed of two or more transmitter/receiver nodes adapted to communicate with each other. Communication systems, such as cell phone systems, use frequency, time and code division multiplexing to ensure only a single transmitter is active at any given instant in time (i.e. for a given set of frequencies and codes). To accomplish a message exchange between nodes, each node is adapted to selectively switch between transmit and receive modes by local node control.
Wireless data communications systems, such as conventional radio frequency systems, provide data communications by modulating, or coding, data signals onto a carrier frequency(s). However, other types of wireless communication systems are carrier-less and rely on time-based coding for data communications. One such communication system that relies on time-based coding to achieve reliable data communications is Ultra Wide Band (“UWB”).
These UWB systems, unlike conventional radio frequency communications technology, do not use modulated carrier frequencies to transport data. Instead, UWB systems make use of a wide band energy pulse that transports data using both time-based coding and signal polarization. Time-based coding methods include pulse-position, pulse-rate or pulse-width techniques. UWB communication systems do not provide a common clock to the transmitting and receiving nodes. Instead, a low-drift clock is implemented in each transmitter/receiver node, providing a local reference for time-based coding and decoding. Each of these multiple clock domains is subject to short-term time drift, which will exceed the necessary tolerance for accurate UWB data communication system operation after a predictable time period. As a result, precise time-synchronization between the transmitting node and receiving node(s) is imperative in UWB systems to obtain accurate data communications. In order to precisely synchronize receiving node(s) with a transmitting node, UWB systems typically require preambles for each transmitted data frame. However, some applications with potential to benefit from UWB technology cannot be implemented if a preamble is required for each phase of the application. Also, many potential applications for UWB technology are size and energy constrained, such as networks of wireless sensors and controls, which seek to minimize transmission time and to conserve energy.
Existing applications employing UWB technology vary from short-range mono-static radar systems to high speed wireless communications characterized by large amounts of data requiring isochronous signaling, such as real-time voice and video. The signal used for a UWB application providing data communication requires a preamble at the beginning of each transmitted data frame to enable a receiver(s) to synchronize with the time-based coding being transmitted. For successful data communications, the participating nodes must remain in time-synchronization, leaving unused the residual time period during which the nodes retain time-synchronization following initial data communications. Also, the energy consumed to transmit the preamble for existing applications is a significant fraction of the overall energy required to transmit the preamble and subsequent data.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the communication industries for a method to provide collaboration among two or more transmitter/receiver nodes that utilizes the residual period of a clock synchronization mechanism required for time-synchronous communication systems, for multiple purposes, including data communication and multi-static radar applications.
The above-mentioned problems of current wireless communication systems are addressed by embodiments of the present invention and will be understood by reading and studying the following summary and specification. The following summary is made by way of example and not by way of limitation. It is merely provided to aid the reader in understanding some of the aspects of the invention. In one embodiment a method of using a wireless communication system to determine locations is provided. The method includes exchanging communication frames between at least two synchronized nodes in the communication system, wherein each communication frame includes at least one of data signals and radar signals. Determining distances of at least one of nodes and reflective sources based in least in part on at least one of direct and reflected radar signals and determining locations of at least one of the nodes and the reflective sources based on the determined distances.